Many measuring techniques and devices require an accurate measurement of the time of flight of a signal. Any technique or device that measures the time of travel of a wave or signal requires a degree of accuracy in determining the measurement. The application dictates the degree of accuracy required. The quantitative determination of the time of travel of a wave or signal in some situations can be relatively easy, or can be difficult. Thus, the need for an accurate measure of the time of travel with respect to the more difficult applications has led to problems in accurate metering. For example, applications which require measurement precision of less than one period of the signal are difficult. Examples of such metering techniques requiring time-of-flight estimates are flow, level, speed of sound, and acoustic impedance measurements. The accurate measurement of the time of travel of the signal is a basic requirement for an effective technique or device.
The measurement of the time of flight of a sonic or ultrasonic signal has different complications than the measurement of the time of flight of a radar signal. In the example of radar, where the time resolution is long with respect to the period of the signal, the time of flight can be readily measured by viewing the envelope of the returning energy. The measurement when the time resolution is short with respect to the period of the signal is more difficult.
Such prior devices and methods which can be used with or are associated with the present invention include, for example, U.S. Pat. No. 2,724,269 to Kalmus entitled "Apparatus for Measuring Flow," U.S. Pat. No. 4,646,575 to O'Hair and Nolan entitled "Ultrasonic Flow Meter" and U.S. Pat. No. 5,178,018 to Gill entitled "System for Measuring the Time for a Signal to Pass Between Two Spaced Points in a Fluid." With respect to U.S. Pat. No. 2,724,269, the phase shift of a signal is associated with a reference. The '269 patent describes periodically interchanging the transmitter and the receiver. U.S. Pat. No. 4,646,575 describes using multiple paths of ultrasonic energy which paths are angularly disposed with regard to the flow and each other. U.S. Pat. No. 5,178,018 provides a system applicable for use in continuous-state wave applications. A phase shift is measured at a particular point in time and used to determine the time for the signal to pass between two spaced points.
It is, therefore, a feature of the present invention to provide method and apparatus for measuring accurately the time of flight of a signal where the period of the signal is long compared to the required time resolution.
A feature of the present invention is to provide an method and apparatus for enhancing the ability to select a specific part of the associated waveform for determining the time of flight of the signal.
Another feature of the present invention is to provide an method and apparatus which enhances the ability to process a signal to determine the time of flight of a signal.
Yet another feature of the present invention is to provide an apparatus for measuring the time of flight of a signal which measurement is not compromised by noise generated by turbine meters, compressors and valves.
Another feature of the present invention is the ability to accurately measure the time of flight of a signal, transmitted from, for example, an ultrasonic transmitter, when the signal is corrupted by the presence of noise.
Yet still another feature of the present invention is to provide an method and apparatus for measuring the time of flight of a signal which method and apparatus is adaptive to signal changes caused by external influences.
Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will become apparent from the description, or may be learned by practice of the invention. The features and advantages of the invention may be realized by means of the combinations and steps particularly pointed out in the appended claims.